Aquifers are underground layers of water-bearing, permeable rock, rock fractures, or unconsolidated materials such as gravel, sand, or silt. They are natural reservoirs that store groundwater, which can be extracted using a water well. Aquifers vary greatly in their characteristics, and the study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Some key features of aquifers include:

• Sustainability: Water from aquifers can be sustainably harvested through the use of qanats, which are underground tunnels that tap into the aquifer.

• Classification: Aquifers can be classified as saturated versus unsaturated, aquifers versus aquitards, confined versus unconfined, isotropic versus anisotropic, porous, karst, or fractured, and transboundary aquifer.

• Types: Aquifers come in two types: alluvial aquifers, which are formed in the space between porous materials such as sand, gravel, silt, or clay, and confined aquifers, which are beneath a rock layer that does not allow water to permeate in measurable amounts.

Aquifers are important sources of water for drinking, commercial, industrial, or agricultural purposes. They are natural filters that trap sediment and other particles and provide natural purification of the groundwater flowing through them. However, challenges for using groundwater include overdrafting, groundwater-related subsidence of land, groundwater becoming saline, and groundwater pollution.